o-Nitroaromatic acids are used in a variety of applications. For example, o-nitrobenzoic acid is a high value, low volume, specialty chemical which is used primarily as a corrosion inhibitor. However, anthranilic acid (2-aminobenzoic acid) can be prepared by hydrogenation of o-nitrobenzoic acid. Anthranilic acid has applications in dyes, pharmaceuticals, perfumes, pigments, flavors, and as an antioxidant in greases and lubricating oils. An economic low-cost method of preparing o-nitrobenzoic acid accordingly is of economic value to prepare anthranilic acid.
The most common route to form anthranilic acid consists of a Hoffman degradation of phthalamic acid alkali metal salt, or phthalimide. Anthranilic acid is also produced by the reduction of o-nitrobenzoic acid, which is formed from the oxidation of o-nitrotoluene with permanganate, or with chromic acid and sulfuric acid. An alternative method for the formation of anthranilic acid is the treatment of isatin with hot 40% potassium hydroxide.
2-Nitroterephthalic acid is also a useful o-nitroaromatic acid. Applications include its use as a corrosion inhibitor, heat transfer agent, polymerization inhibitor and as a catalyst for curing inks and coatings. 2-Nitroterephthalic acid is also a useful chemical intermediate. For example, the nitro group may be hydrogenated to an amino group to prepare 2-amino-terephthalic acid. Alternatively, the nitro group may be readily displaced by active nucleophilic agents such as methoxide or phenoxide. The nucleophilic displacement of the nitro group with, for example, a methoxy group would produce 2-methoxyterephthalic acid whereas the displacement with a phenoxy group would produce 2-phenoxy-terephthalic acid. These and other 2-substituted terephthalic acid compounds that can be prepared from 2-nitroterephthalic acid are useful monomers for the formulation of heat processible polymeric materials. Furthermore, we have determined that 2-nitroterephthalic acid is not readily prepared by the nitration of terephthalic acid.
It has been known that while meta and para-nitrotoluene are readily oxidized by means of molecular oxygen in the presence of bromine and a heavy metal oxidation catalyst to the corresponding nitrobenzoic acid, the corresponding ortho isomer, namely ortho-nitrotoluene, is unexpectedly resistant to oxidation by this process. Similarly, other ortho-nitrosubstituted aromatics, such as 2-nitro-paraxylene are unexpectedly resistant to oxidation. It is particularly surprising that ortho-nitrotoluene does not oxidize in this process, since there is no difficulty observed in the chemical oxidation of ortho-nitrotoluene. For example, potassium dichromate and sulfuric acid have been employed to give high yields of ortho-nitrobenzoic acid (see Org. Chem. Ind. (U.S.S.R.), 7, p. 379 (1940)) and similar results have been obtained with boiling aqueous permanganate, e.g. as reported by M. Boetius, Berichte Deutsche Chemische Gesellschaft (Ber., 68B, p. 1924 (1935)). It has also been reported that when boiled with a sodium hydroxide solution, orthonitrotoluene exhibits the phenomena of autoxidation and reduction and yields anthranilic acid (Kirk-Othmer, Encyl. Chem. Tech., 15, 3rd, p. 926).
General background references using the Hoffmann degradation of phthalamide derivatives to prepare anthranilic acid or the oxidation of ortho-nitrotoluene with expensive permanganate and chromic acid are disclosed in the following patents: Romanian Patent No. 74,453, Romanian Patent No. 79,909, U.S. Pat. No. 4,233,459, U.S. Pat. No. 4,276,433, U.S. Pat. No. 3,847,974, U.S. Pat. No. 3,322,820, U.S. Pat. No. 3,882,171, French Patent No. 2,512,844, and Japanese Patent No. 57,026,652.
Various attempts have been made in the past to oxidize ortho-nitrotoluene by autoxidation wherein the reaction is catalyzed by transition metal and bromide ions in acetic acid. As reported by R. Hasegawa and Y. Kamiya, Bul. Chem. Soc. Japan, 51, No. 5, 1490-94 (1978), proceeding only to ortho-nitroacetophenone, ortho-nitroalkylbenzenes, except ortho-nitroethylbenzene, shows a great resistivity against the oxidation, with the authors describing the results as explained by the retardation effect due to inactive free radicals and also to the various reduced products resulting from the intra-molecular reaction of various nitro groups and active radicals.
An object of this invention accordingly is to provide a process for the oxidation of an ortho-nitroaromatic to an ortho-nitroaromatic acid in high yield. These orthonitroaromatics have a group positioned next to the nitro group on the aromatic ring wherein the group is oxidizable to a carboxylic acid. Another object of this invention is to prepare ortho-nitroaromatic acids by the oxidation of an ortho-nitroaromatic by means of molecular oxygen in the presence of bromine and a heavy metal oxidation catalyst. It is further an object of this invention to prepare ortho-nitroaromatic acids from an ortho-nitroaromatic compound by autoxidation in the presence of bromine and a heavy metal oxidation catalyst and a co-oxidizable compound.
A further object is to provide an oxidation process employing a solvent medium from which ortho-nitrobenzoic acid can be obtained directly and in high yield and high purity. These and other objects of this invention will be apparent from the ensuing description thereof.